Question

As the acidity of a solution increases
1. Hydrogen ion concentration increases
2. Hydroxyl ion concentration decreases
3. Both 1 and 2
4. None of the above.

Answer 1

Hint: Dissociation of acid takes place in the following method and there is a release of Hydrogen ion. This increases acidity of solution.
${H^ + } = Hydrogen\,\,\,ion$
Dissociation of base takes place in the following method and there is a release of Hydroxyl ion. This increases basicity of solution.
$O{H^ - } = Hydroxyl\,\,ion$

Complete step by step solution:
Let us first understand the terms used in the question.
Acidity is the measure of the strength of an acid or we can say, by measuring the acidity, we come to know how strong the acid is.
General dissociation reaction of an acid:
${H^ + } = Hydrogen\,\,\,ion$
 Let us notice how acids disassociate with an example
$HCl\xrightarrow{{}}{H^ + } + C{l^ - }$
Where: $HCl = Hydrochloric\,acid,{H^ + } = proton/Hydrogen\,ion,C{l^ - } = Chloride\,ion$
Every acid when disassociate, it gives ${H^ + } = Hydrogen\,\,\,ion$
So we can say, the strength of an acid can be measured by the concentration of ${H^ + }$ ions in the solution, More the number of ${H^ + }$ , more is the acidity.
Basicity is the measure of the strength of a base, by this value we come to know how strong the base is.
General dissociation reaction of a base:

$O{H^ - } = Hydroxyl\,\,ion$
Let us see how base disassociate with an example:
$NaOH\xrightarrow{{}}N{a^ + } + O{H^ - }$
$NaOH = Sodium\,Hydroxide,N{a^ + } = Sodium\,ion,O{H^ - } = Hydroxyl\,ion$
Every acid when disassociated, it gives $O{H^ - } = Hydroxyl\,\,ion$ .
Hence we can say that the strength of a base or basicity increases when there are more numbers of $O{H^ - }$ ions.
We can reverse this to say that, The acidity decreases when the number of $O{H^ - }$ ions increase.
Hence, Option 3 is correct.

Note:
-The concept of the number of ${H^ + }$ and $O{H^ - }$ ions being used to determine the strength of a solution is only valid for Arrhenius and Bronsted Acid and base.
-Acidity of compounds like $AlC{l_3}$ cannot be determined on the basis of ${H^ + }$concentration. These are Lewis acid and bases.